In some auto-focus cameras the focussing system operates by directing a beam of light, usually a pulsed beam of infra-red light, onto the subject and sensing the reflected light through a detector lens. The light is usually sensed by a detector comprising a photo-transistor, photo-diode or photo-darlington or an array of such devices. The balance across the detector array or the strength of the reflected light from the subject reaching the detector depends on the distance of the subject from the detecting lens. The differential output or the amplitude of the output from the detector can then be used to control the focusing of the picture lens. Such systems are well known in the art.
A difficulty with such systems is that the detector must work in widely varying degrees of overall illumination, from moonlight to bright sunshine, where the strength of the light may vary by a factor of 1 million. In low light conditions the detector becomes slow to react to changes in illumination and the output signal is low, reducing the sensitivity of the system.